As a consequence of the increasing complexity of modern mining machinery and systems, more time is consumed on repair following a breakdown. Equipment breakdowns cause lengthy and costly delays in the operation of mines. PROGNOS is a rule-based expert system for fault diagnosis of the transmission system of load-haul-dump (LHD) vehicles in Kurna mine, Sweden. This paper presents the problem approach and discusses the formulation of the knowledge base and the software implementation. The use of expert systems for fault diagnosis of mining machinery can be a cost and time effective approach of minimizing repairs in the mines